1. Field of the Invention
The present invention relates to a charged particle beam apparatus and in particular, to a charged particle beam apparatus having a function of controlling the brightness or contrast of an image.
2. Background Art
As a function of automatically controlling the brightness or contrast of an image obtained by a scanning electron microscope (SEM), there is automatic brightness/contrast control (ABCC) or automatic contrast/brightness control (ACBC).
JP-A-2001-148230 discloses means for automatically controlling the brightness/contrast using the maximum and minimum values of a detected electrical signal. In addition, JP-A-2001-148230 and JP-A-2001-243907 disclose a method of automatically controlling the brightness/contrast by calculating the histogram and the cumulative frequency distribution of an image. This is a method of calculating the numerical evaluation value by appropriately processing an image and making the evaluation value converge while changing the parameters of an apparatus such that the evaluation value approaches a setting value.
When controlling the brightness/contrast automatically, it is desirable to perform pre-processing for removing noise on an image or an electrical signal. For example, in the case of control using a digital image, it is desirable to remove noise components by applying an image processing filter, such as a smoothing filter or Gaussian filter. In addition, it may be considered to remove noise by integrating a plurality of frame images obtained on the basis of multiple beam scans. In addition, in the case of control using an electrical signal, it is considered to use a low pass filter in order to remove noise components.
All of the above methods can be said to be effective techniques from the point of view of image quality improvement. As pre-processing for performing the ABCC, however, it is necessary to set the appropriate conditions in consideration of the following points.
For example, if the number of integrated frames is small, noise removal is not sufficient. On the contrary, if the number of integrated frames is large, a long time is required for image acquisition. In addition, when an image processing filter is used, for example, if the radius of a smoothing filter is small, subsequent contrast control may not be appropriately performed. If the radius of the smoothing filter is large, there is a possibility that the contrast enhancement will become excessive. In addition, if the cutoff frequency of a low pass filter is too high, contrast control may not be appropriately performed. In addition, if the cutoff frequency of the low pass filter is too low, there is a possibility that the contrast enhancement will become excessive. JP-A-2001-148230 and JP-A-2001-243907 do not disclose a method for realizing the optimization of the above pre-processing.
A charged particle beam apparatus to select appropriate pre-processing conditions when performing automatic brightness/contrast control will be described below.